This invention relates to a method and apparatus for nuclear well logging and more particularly to a new and improved gamma ray detector for measuring gamma radiation from formations surrounding a borehole.
Various methods and apparatus have been utilized in the well logging art to study the radioactive properties of subsurface formations, both where the radiation is natural and where it is artificially induced. Logs of such properties aid in the study of the nature of the subsurface formations, particularly in exploration for minerals and hydrocarbon deposits. One such method and apparatus utilizing the measurement of gamma radiation to identify subsurface formations is disclosed in U.S. Pat. No. 3,940,610 to Dennis et al. Such patent discloses a borehole logging tool having a gamma ray detector of the scintillation type which cooperates with a photomultiplier tube for detecting natural gamma radiation. The scintillation detector is a crystal of sodium iodide activated with thallium. The sodium iodide gives off quanta of light energy called photons, the intensity of which is proportional to the energy of the gamma ray interaction in the sodium iodide crystal. The photomultiplier tube responds to these photons to produce a succession of electrical pulses having magnitudes proportional to the energy of the gamma rays. These pulses are amplified and sent uphole to a surface recording system.